Method for determining the current position of the heads of vehicle occupants

ABSTRACT

The invention relates to a method for determining the current position (A, B, C, D) of a head ( 9 ) of an occupant ( 8 ) in the passenger compartment ( 2 ) of a motor vehicle ( 1 ), said head moving toward an automatic dynamic disabling zone ( 6 ) in front of an airbag module ( 5 ). To this end, the invention makes use of the idea that the best position for a measurement with regard thereto is the point in space where the ideal direction of movement ( 14 ) of the head ( 9 ) is perpendicular to an ideal line of sight ( 17 ) of the camera ( 160.  The measurement is then preferably carried out when the geometric center ( 10 ) of the head ( 9 ) crosses this point. In a preferred embodiment, the calculation of the actual movement vector of the head ( 9 ) is taken as a basis, said head being preferably perpendicular in a current line of sight ( 18 ) of the camera ( 16 ). The invention advantageously increases the potential for protecting an occupant ( 8 ) in a motor vehicle ( 1 ). It is thus suited, in particular, for use in occupant protection systems of modern motor vehicles ( 1 ).

The present invention relates to a method for determining the currentposition (A, B, C, D . . . ) of a head of an occupant in the passengercompartment of a motor vehicle, said head moving toward a dynamicdisabling zone in front of an air bag module.

Future restraint systems in motor vehicles will have to take intoaccount the current position of the heads of vehicle occupants, in orderto achieve the optimum protective effect during airbag activation. Thesedevelopments are also driven by legislative initiatives, for example inthe US by legislative initiative FMVSS 208.

A important element of such legislation is the automatic dynamicdisabling zone in front of the airbag module (hereafter referred to asthe critical out of position zone or COOP. The limit of this zone istypically located at a distance of around 10 cm from the airbag moduletoward the occupant. If during an accident (e.g. as a result of totalbrake application) the head of the vehicle occupant is thrown toward theairbag module, the legislation requires the airbag to be disabled beforethe head or torso enters the COOP zone. A critical factor here is thedistance from the part of the head (generally the face) or torso(generally the chest) facing the airbag to the airbag module.

An important criterion for optimizing this function is the size of theso-called switch zone. On the one hand the airbag must be deactivatedbefore the head enters the COOP zone but on the other hand the occupantmust not be denied any protection potential at an adequate distance fromthe airbag. The switch zone should therefore be as small as possible.The size of the switch zone is typically around a few centimeters.

Contactless systems and primarily optical systems have been developed tomeasure the distance of the head of the occupant from the airbag module.These optical systems are preferably 3D cameras, which operate accordingto the stereoscopic method, the pencil of light method or TOF (time offlight).

Such image acquisition units are typically integrated up in the roofmodule between the vehicle seats (see FIGS. 1 and 2 below). Theadvantage of this integration position is that under normalcircumstances the camera cannot easily be covered, e.g. by occupantsreading newspapers. One disadvantage is however that when the headenters the COOP zone, the camera cannot see the part of the head facingthe airbag module, i.e. generally the face. In this instance the imageacquisition unit is looking at the back of the occupant's head.

It is known that this problem can be resolved by assuming a mean headsize. However this assumption is subject to not insignificant error inthe case of bouffant hairstyles, head coverings, etc. The assumption nolonger applies in such cases. The switch zone must therefore be bigenough to cover most scenarios given this uncertainty.

FIG. 3 illustrates this situation. In head position A (seated normally)the camera sees the face of the occupant but not the hair. In headposition B (face at limit of COOP zone) the camera looks at the back ofthe occupant's head and sees the hair but no longer the face of theoccupant.

FIG. 4 shows that if a mean head size is assumed, even a slightlybouffant hairstyle can result in the camera determining a positionoutside the COOP even though the head has already entered the COOP.

The object of the invention is to specify an improved method, whichavoids the above disadvantages, and a device for the exact determinationof the position of the heads of vehicle occupants. In particular theinvention should allow the smallest possible switch zone to be achieved,so as not to deny the occupant any protection potential at an adequatedistance from the airbag module.

This object is achieved by the independent claims. Advantageousembodiments and developments, which can be deployed individually or incombination with each other, are set out in the dependent claims.

A first method according to the invention for determining the exactposition (A, B, C, D . . . ) of a head of an occupant in the passengercompartment of a motor vehicle, said head moving toward an automaticdynamic disabling zone in front of an airbag module, using an imageacquisition unit with an ideal line of sight, which is essentiallyperpendicular to the ideal direction of movement of the occupant, ischaracterized in that the image acquisition unit is used at leastcyclically to record an image scenario including the occupant, i.e. animage area including the occupant, in the passenger compartment of themotor vehicle and to detect image information relating to the occupant;in the respective current scenario image both the position of thegeometric center of the head and the apparent size of the head aredetermined, in particular measured in the direction of movement; therespective current lines of sight of the image acquisition unit aredefined as a vector from a defined position of the image acquisitionunit to the respective current position of the geometric center of thehead; the respective current angle α between the ideal line of sight andthe current lines of sight of the image acquisition unit are calculated;the respective current values for angle α and apparent size of the headare stored in a storage unit; and the value from the storage unit forwhich the absolute sum of the difference between angle α and 0° wasminimal is always assumed to be the size of the head.

Contrary to the prior art therefore instead of a mean head size, anapparent and therefore ultimately real head size is determined. Thenduring the for example parallel position calculations the head sizes,which are closest to the actual head size and in the best instance mayeven correspond to it, are selected from the set of head sizes thusdetermined and used as the basis for calculation. Using a head sizebased on reality for determination of the current position in front ofthe airbag module advantageously gives a much more exact positiondetermination than is possible with the prior art and therefore allows asignificantly smaller switch zone to be achieved, thereby increasing theprotection potential for the occupant of the vehicle.

The present invention also relates to a method for determining the exactposition (A, B, C, D . . . ) of a head of an occupant in the passengercompartment of a motor vehicle, said head moving toward an automaticdynamic disabling zone in front of an airbag module, using an imageacquisition unit with an ideal line of sight, which is essentiallyperpendicular to the ideal direction of movement of the occupant, theimage acquisition unit being used at least cyclically to record an imagescenario including the occupant, i.e. an image area including theoccupant, in the passenger compartment of the motor vehicle and todetect image information relating to the occupant; both the position ofthe geometric center of the head and the apparent size of the head beingmeasured in the direction of movement in the respective current scenarioimage; the respective current lines of sight of the image acquisitionunit being defined as a vector, from a defined position of the imageacquisition unit to the respective current position of the geometriccenter of the head; the respective current angles β between thedirection of movement of the head and the current lines of sight of theimage acquisition unit being calculated; the respective current valuesfor angle β and the apparent size of the head being stored in a storageunit; and the value from the storage unit for which the absolute sum ofthe difference between angle β and 90° was minimal being assumed to bethe size of the head. This method also has the advantages mentionedabove.

In a preferred embodiment of the invention the image acquisition unit isa 3D camera, which operates according to the stereoscopic method, thepencil of light method, the time of flight method or another equallyappropriate method. Use of a 3D camera, in particular in respect of thelast mentioned method according to the invention, advantageously allowsdetermination of the current position independently of ideal movementdirections and/or lines of sight based on actual movement vectors inthree-dimensional space.

Instead of carrying out a position determination based on an idealdirection of movement, it is therefore preferably proposed according tothe invention that the respective current angle β is calculated betweena current actual movement vector of the head and the current line ofsight vector of the image acquisition unit. This advantageously enhancesthe reliability of the method according to the invention even when anoccupant is sitting with a lateral orientation, as for example occursnormally when looking out of a side window, etc.

Because the respective last or previous value for the position of thegeometric center of the head is preferably stored, the respectivecurrent movement vector can advantageously be calculated in a simplemanner from the stored last and the current 3-dimensional positions ofthe head.

According to the invention the defined position of the image acquisitionunit is preferably based on the center of a lens aperture of the imageacquisition unit, in the case of the stereo camera for example the leftlens.

In a further preferred embodiment of the invention, the imageacquisition unit records images of the relevant scene continuously, atleast 25 images per second, in particular at least 30 images per second,preferably at least 35 images per second, so that film speeds, i.e.online viewing, can actually advantageously be achieved.

The size of the storage unit is such that at least all the measuredvalues of a slow head movement from a first head position (A), i.e. aninitial position, to the relevant second head position (B), namely theposition with the face at the limit of the COOP area, can be stored.

In one expedient embodiment of the invention the storage unit is a ringbuffer store, which is first filled and from then on the oldest value isreplaced by the current value.

It has also proven advantageous for the recordings of the head movementto be filtered using filters such as Kalman filters and/or movementmodels.

It should be noted specifically that instead of determining the headposition, with the image acquisition unit any other appropriate part ofthe body of an occupant can be used as the basis for calculating theposition of said occupant, such as in particular the size of the torsoof the occupant in the motor vehicle.

In a further preferred embodiment of the invention it is proposed thatdifferent views of the occupant be combined to provide a 3D overall viewof the person. This advantageously allows simulation for example of thefront or facial profile of the occupant in front of the plane of theCOOP limit from the overall view and therefore more accurate calculationof its distance from the disabling zone or the airbag module than withthe prior art.

For the sake of completeness, it should be pointed out that theinvention also of course relates to a device corresponding to the methodfor determining the current position (A, B, C, D . . . ) of a head of anoccupant in the passenger compartment of a motor vehicle, said headmoving toward an automatic dynamic disabling zone in front of an air bagmodule, said device being characterized by appropriate means forcarrying out the method as described above.

Particular problems relating to the legislative initiative FMVSS 208mentioned above are known to be posed by the validatability andreproducibility of the head position measurements in the context of thelicensing procedure for a camera system. By improving the head positiondetermination even by a matter of centimeters, the present invention canoffer crucial advantages both here and in the field. By advantageouslyenhancing the protection potential of an occupant in a motor vehiclethus, the present invention is in particular suitable for occupantprotection systems in modern motor vehicles.

The invention is described below with reference to different exemplaryembodiments and in conjunction with the drawing, in which:

FIG. 1 shows a schematic diagram of a top view of a typical integrationlocation for an image acquisition unit in the passenger compartment of amotor vehicle;

FIG. 2 shows a schematic diagram of a side view of the integrationlocation according to FIG. 1;

FIG. 3 shows a schematic diagram of the measurement of the headpositions of an occupant assuming mean head sizes;

FIG. 4 shows a schematic diagram of the incorrect measurement based onthe assumption according to FIG. 3 of the head positions of an occupantwith a bouffant hairstyle for example;

FIG. 5 shows a schematic diagram of a first exemplary embodiment of themeasurement of head positions according to the invention; and

FIG. 6 shows a schematic diagram of a second exemplary embodiment of themeasurement of head positions according to the invention.

FIG. 1 shows a top view of the typical integration location of an imageacquisition unit 16 in the passenger compartment 2 of a motor vehicle 1.

FIG. 2 shows a side view of the diagram in FIG. 1.

An image acquisition unit 16, for example a 3D camera, is generallyintegrated in a module up in the roof liner 20 between the vehicle seats3. An airbag module 5 is housed in front of the seats 3, typically inthe dashboard 4. An automatic dynamic COOP disabling zone 6 in front ofthe airbag 5 serves for the statutorily required protection of anoccupant 8 from injury by the airbag 5, if a minimum distance therefromis not observed. The advantage of the integration location 20 describedabove for the camera 16 is that under normal circumstances it cannoteasily be covered, for example by occupants 8 reading newspapers. It ishowever a disadvantage that when the head 9 enters the COOP zone 6, thecamera 16 cannot see the part of the head 9 facing the airbag module 5,generally the face 12. In this instance the camera 16 is looking at theback of the head of the occupant 9.

It is known that this problem can be resolved by assuming a mean headsize. This assumption is however subject to not insignificant error inthe case of bouffant hairstyles, head coverings, etc. The assumption nolonger applies in such cases. A switch zone 7 in front of the COOP zonemust therefore be big enough to cover most scenarios given thisuncertainty.

FIG. 3 illustrates this situation, i.e. the measurement of headpositions of an occupant 8 assuming mean head sizes. In head position A(seated normally) the camera 16 sees the face 12 of the occupant but notthe hair. In head position B (face at limit of COOP zone) the camera 16looks at the back of the head of the occupant 8 and sees the hair but nolonger the face 12 of the occupant 8.

FIG. 4 shows the incorrect measurement based on the assumption accordingto FIG. 3. If a mean head size is assumed, even a slightly bouffanthairstyle can result in the head 9 entering the COOP zone 6 but thecamera still determining a position outside the COOP zone 6.

FIG. 5 shows a first exemplary embodiment of the measurement of headpositions according to the invention in principle. To this end the size11 of the head 9 of the occupant 8 is determined or measured asaccurately as possible, particularly at head position C. The inventionhereby utilizes the idea that the best position for a measurement withregard thereto is the point in space, at which the ideal direction ofmovement 14 of the head 9 is perpendicular to an ideal line of sight 17of the camera 16. The measurement is then preferably carried out whenthe geometric center 10 of the head 9 crosses this point.

A method for determining the current size 11 of the head 9 for exampleimplemented in an algorithm can for example be as follows:

Known methods of image processing are used in the respective current 3Dimage to measure the position of the geometric center 10 of the head 9,as well as the apparent size 11 of the head 9 in the direction ofmovement 14.

The current line of sight 18 of the camera 16 is defined as a vector,from a defined camera position 19, for example the center of the lensaperture, in the case of a stereo camera 16 for example the left lens,to the position of the geometric center 10 of the head 9.

A current angle α between the ideal line of sight 17 and the currentlines of sight 18 of the 3D camera 16 is then calculated according toknown methods of linear algebra. Alternatively or even cumulatively as aplausibility check for example, a current angle β can be calculatedbetween the ideal direction of movement 14 of the head 9 and the currentlines of sight 18 of the image acquisition unit 16.

The respective current values for angles α and β and apparent head size11 are then stored, preferably in a ring buffer store (not shown), inwhich the oldest value is always replaced by the current value. The sizeof the ring buffer store is preferably dimensioned such that at leastall the measured values of a slow head movement 14 from head position Ato head position B can be stored.

The value from the current ring buffer store for which the absolute sumof the difference between angle α and 0° or angle β and 90° was minimalis always assumed to be the size of the head.

FIG. 6 shows a second exemplary embodiment of a preferred measurement ofhead positions according to the invention.

First known image processing methods are used in the respective current3D image to measure the position of the geometric center 10 of the head9, as well as the apparent size 11 of the head 9 in the direction ofmovement 14.

The current line of sight 18 of the camera 16 is also defined as avector, from a defined camera position 19, for example the center of thelens aperture, in the case of a stereo camera 16 for example the leftlens, to the position of the geometric center 10 of the head 9.

Unlike in FIG. 5, the respective last, i.e. previous, values are nowstored for the position 10, so that an actual movement vector 15 of thehead 9 can be calculated from the stored last and current 3-dimensionalhead position 10.

The current angle β between the actual movement vector 15 of the head 9and the current lines of sight 18 of the 3D camera 16 are thencalculated according to known methods of linear algebra.

The respective current values for angle β and apparent head size 11 arethen stored, again preferably in a ring buffer store (not shown), inwhich the oldest value is always replaced by the current value.

The value from the current ring buffer store for which the absolute sumof the difference between angle β and 90° was minimal, i.e. the geometrywas closest to the “perpendicular” condition, is always assumed to bethe size of the head.

It should be noted that the methods described above can be stabilizedfurther by further measures. For example the head movement 14 or 15 canbe filtered using known filters and movement models (Kalman filters).

The above methods can also advantageously be applied correspondingly tothe determination of the distance of the torso 13 of the occupant fromthe airbag module 5. The size of the torso 13 of the occupant 8 is thendetected using the 3D camera instead of the size 11 of the head 9.

The above methods can be developed still further by combining thedifferent views of the person 8, which result from the differentpositions A, B, C, . . . of the person 8 in relation to the camera 16,to form a three-dimensional overall view of the person 8, e.g. frontview A with person sitting normally plus side view C with person 8 nextto camera 16 plus rear view B with person 8 bending.

All partial views A, B, C, D, . . . , recorded during the forwardmovement of the person 8 by the 3D camera 16 and buffered in the ringbuffer store, can also be used for the combination of partial views A,B, C, . . . of the person 8.

When the person 8 is close to the COOP zone 6, practically only the rearof the person is visible to the 3D camera 16. The non-visible front 12of the person 8 can now be simulated from the overall view determinedabove and the distance from the airbag module 5 can be calculated moreaccurately.

The present invention advantageously enhances the protection potentialof an occupant 8 in a motor vehicle 1. It is therefore suitable inparticular for occupant protection systems in modern motor vehicles 1.

1-14. (canceled)
 15. A method for determining a current position of ahead of an occupant in a passenger compartment of a motor vehicle, thehead moving toward an automatic dynamic disabling zone in front of anairbag module, which comprises the steps of: providing an imageacquisition unit having an ideal line of sight being substantiallyperpendicular to an ideal direction of movement of the occupant;recording an image scenario in the passenger compartment of the motorvehicle containing the occupant at least cyclically with the imageacquisition unit and image information relating to the occupant beingdetected; determining both a position of a geometric center of the headand an apparent size of the head in the ideal direction of movement in arespective current scenario image; defining current lines of sight ofthe image acquisition unit as a vector, from a defined position of theimage acquisition unit to a respective current position of the geometriccenter of the head; calculating respective current angles values betweenthe ideal line of sight and the current lines of sight of the imageacquisition unit; storing the respective current angles values and theapparent size of the head in a storage unit; and using the apparent sizeof the head stored in the storage unit at which an absolute sum of adifference between the respective current angle values and 0° wasminimal to be an actual size of the head.
 16. The method according toclaim 15, which further comprises using a 3D camera as the imageacquisition unit, the 3D camera operating according to a method selectedfrom the group consisting of a stereoscopic method, a pencil of lightmethod, and a time of flight method.
 17. The method according to claim15, which further comprises basing the defined position of the imageacquisition unit on a center of a lens aperture of the image acquisitionunit.
 18. The method according to claim 17, which further comprisesusing a stereo camera as the image acquisition unit and basing thedefined position on a center of a left lens aperture of the stereocamera.
 19. The method according to claim 15, which further comprisesrecording continuously, via the image acquisition unit, images of arelevant scenario, at least 25 images per second.
 20. The methodaccording to claim 15, which further comprises dimensioning a size ofthe storage unit such that at least all measured values of a slow headmovement from a first head position to a second head position can bestored.
 21. The method according to claim 15, which further comprisesusing a ring buffer store as the storage unit, the ring buffer storebeing first filled and from then on an oldest value is always replacedby a current value.
 22. The method according to claim 15, which furthercomprises filtering recordings of a head movement by filters and/ormovement models.
 23. The method according to claim 15, which furthercomprises combining different views of the occupant for forming a 3Doverall view of the occupant.
 24. The method according to claim 23,which further comprises: simulating a front of the occupant for the 3Doverall view; and calculating a distance between the automatic dynamicdisabling zone or the airbag module and the front of the occupant. 25.The method according to claim 15, which further comprises recordingcontinuously, via the image acquisition unit, images of a relevantscenario, at least 30 images per second.
 26. The method according toclaim 15, which further comprises recording continuously, via the imageacquisition unit, images of a relevant scenario, at least 35 images persecond.
 27. The method according to claim 24, which further comprisessimulating a facial profile of the occupant.
 28. A method fordetermining a current position of a body part of an occupant in apassenger compartment of a motor vehicle, the body part moving toward anautomatic dynamic disabling zone in front of an airbag module, whichcomprises the steps of: providing an image acquisition unit having anideal line of sight being substantially perpendicular to an idealdirection of movement of the occupant; recording an image scenario inthe passenger compartment of the motor vehicle containing the occupantat least cyclically with the image acquisition unit and imageinformation relating to the occupant being detected; determining both aposition of a geometric center of the body part and an apparent size ofthe body part in the ideal direction of movement in a respective currentscenario image; defining current lines of sight of the image acquisitionunit as a vector, from a defined position of the image acquisition unitto a respective current position of the geometric center of the bodypart; calculating respective current angles values between the idealline of sight and the current lines of sight of the image acquisitionunit; storing the respective current angles values and the apparent sizeof the body part in a storage unit; using the apparent size of the bodypart stored in the storage unit at which an absolute sum of a differencebetween the respective current angle values and 0° was minimal to be anactual size of the body part.
 29. The method according to claim 28,which further comprises determining a size of a torso as the body part.30. A method for determining a current position of a head of an occupantin a passenger compartment of a motor vehicle, the head moving toward anautomatic dynamic disabling zone in front of an airbag module, whichcomprises the steps of: providing an image acquisition unit having anideal line of sight being substantially perpendicular to an idealdirection of movement of the occupant; recording an image scenario inthe passenger compartment of the motor vehicle containing the occupantat least cyclically with the image acquisition unit and imageinformation relating to the occupant being detected; determining both aposition of a geometric center of the head and an apparent size of thehead in a ideal direction of movement in a respective current scenarioimage; defining current lines of sight of the image acquisition unit asa vector, from a defined position of the image acquisition unit to arespective current position of the geometric center of the head;calculating respective current angles between the ideal direction ofmovement of the head and the current lines of sight of the imageacquisition unit; storing the respective current values for angles andthe apparent size of the head in a storage unit; and using the apparentsize of the head stored in the storage unit at which an absolute sum ofa difference between the respective current angle values and 90° wasminimal to be an actual size of the head.
 31. The method according toclaim 30, which further comprises using a 3D camera as the imageacquisition unit, the 3D camera operating according to a method selectedfrom the group consisting of a stereoscopic method, a pencil of lightmethod, and a time of flight method.
 32. The method according to claim30, which further comprises basing the defined position of the imageacquisition unit on a center of a lens aperture of the image acquisitionunit.
 33. The method according to claim 32, which further comprisesusing a stereo camera as the image acquisition unit and basing thedefined position on a center of a left lens aperture of the stereocamera.
 34. The method according to claim 30, which further comprisesrecording continuously, via the image acquisition unit, images of arelevant scenario, at least 25 images per second.
 35. The methodaccording to claim 30, which further comprises dimensioning a size ofthe storage unit such that at least all measured values of a slow headmovement from a first head position to a second head position can bestored.
 36. The method according to claim 30, which further comprisesusing a ring buffer store as the storage unit, the ring buffer storebeing first filled and from then on an oldest value is always replacedby a current value.
 37. The method according to claim 30, which furthercomprises filtering recordings of a head movement by filters and/ormovement models.
 38. The method according to claim 30, which furthercomprises combining different views of the occupant for forming a 3Doverall view of the occupant.
 39. The method according to claim 38,which further comprises: simulating a front of the occupant for the 3Doverall view; and calculating a distance between the automatic dynamicdisabling zone or the airbag module and the front of the occupant. 40.The method according to claim 30, which further comprises recordingcontinuously, via the image acquisition unit, images of a relevantscenario, at least 30 images per second.
 41. The method according toclaim 30, which further comprises recording continuously, via the imageacquisition unit, images of a relevant scenario, at least 35 images persecond.
 42. The method according to claim 39, which further comprisessimulating a facial profile of the occupant.
 43. A method fordetermining a current position of a head of an occupant in a passengercompartment of a motor vehicle, the head moving toward an automaticdynamic disabling zone in front of an airbag module, which comprises thesteps of: providing an image acquisition unit having an ideal line ofsight being substantially perpendicular to an ideal direction ofmovement of the occupant; recording an image scenario in the passengercompartment of the motor vehicle containing the occupant at leastcyclically with the image acquisition unit and image informationrelating to the occupant being detected; determining both a position ofa geometric center of the head and an apparent size of the head in aideal direction of movement in a respective current scenario image;defining current lines of sight of the image acquisition unit as avector, from a defined position of the image acquisition unit to arespective current position of the geometric center of the head;calculating respective current angles between current actual movementvectors of the head and the current lines of sight of the imageacquisition unit; storing the respective current values for angles andthe apparent size of the head in a storage unit; and using the apparentsize of the head stored in the storage unit at which an absolute sum ofa difference between the respective current angle values and 90° wasminimal to be an actual size of the head.
 44. The method according toclaim 43, which further comprises: storing a respective last value forthe position of the geometric center of the head; and calculating arespective current movement vector from the stored last and current3-dimensional positions of the head.
 45. A device for determining acurrent position of a head of an occupant in a passenger compartment ofa motor vehicle, the head moving toward an automatic dynamic disablingzone in front of an airbag module, the device comprising: an imageacquisition unit having an ideal line of sight being substantiallyperpendicular to an ideal direction of movement of the occupant, saidimage acquisition unit programmed to: record an image scenario in thepassenger compartment of the motor vehicle containing the occupant atleast cyclically with the image acquisition unit and image informationrelating to the occupant being detected; determine both a position of ageometric center of the head and an apparent size of the head in theideal direction of movement in a respective current scenario image;define current lines of sight of the image acquisition unit as a vector,from a defined position of the image acquisition unit to a respectivecurrent position of the geometric center of the head; calculaterespective current angles values between the ideal line of sight and thecurrent lines of sight of the image acquisition unit; store therespective current angles values and the apparent size of the head in astorage unit; and use the apparent size of the head stored in thestorage unit at which an absolute sum of a difference between therespective current angle values and 0° was minimal to be an actual sizeof the head.
 46. A device for determining a current position of a headof an occupant in a passenger compartment of a motor vehicle, the headmoving toward an automatic dynamic disabling zone in front of an airbagmodule, the device comprising: an image acquisition unit having an idealline of sight being substantially perpendicular to an ideal direction ofmovement of the occupant, said image acquisition unit programmed to:record an image scenario in the passenger compartment of the motorvehicle containing the occupant at least cyclically with the imageacquisition unit and image information relating to the occupant beingdetected; determine both a position of a geometric center of the headand an apparent size of the head in a ideal direction of movement in arespective current scenario image; define current lines of sight of theimage acquisition unit as a vector, from a defined position of the imageacquisition unit to a respective current position of the geometriccenter of the head; calculate respective current angles between theideal direction of movement of the head and the current lines of sightof the image acquisition unit; store the respective current values forangles and the apparent size of the head in a storage unit; and use theapparent size of the head stored in the storage unit at which anabsolute sum of a difference between the respective current angle valuesand 90° was minimal to be an actual size of the head.